This invention relates to an optical potentiometer and, more particularly, to a potentiometer having two light responsive devices only one of which is energized at a time and where a dwell period may be provided between the times that the two light responsive devices receive energizing light.
Variable resistors are used extensively for providing control and setpoint functions in electrical circuits. For example, variable resistors are used as motor speed and light intensity controls, as trimming and compensating resistors in electrical apparatus, and for setpoint control in electrical condition controlling circuits. Most prior art variable resistors comprise a fixed resistance element and a slidable wiper arm which is in mechanical contact with the fixed resistance and wipes over the resistance to provide a variable output voltage between the voltages to which the ends of the fixed resistance are connected.
In such devices, good electrical contact between the wiper arm and the fixed resistance must be maintained to prevent degradation of the output voltage supplied by the wiper arm and to maintain the connection between the fixed resistance and the circuit to which the wiper arm is connected. The friction resulting from the wiping motion of the wiper arm over the fixed resistance results in wearing of the contact point between the wiper arm and the fixed resistance potentially weakening the contact point and leading to a break and subsequent open circuit condition. Moreover, this wearing and any oxydation to which the contact point may be subjected tends to generate undesirable electrical noise.
To avoid the problems of a direct electrical contact between the wiper arm and the fixed resistance, the prior art turned to optical potentiometers. However, the optical potentiometers of the prior art duplicates the same operating characteristics of resistance type potentiometers and, therefore, have all of the built-in inflexibilities of such resistance type potentiometers. Specifically, in resistance type potentiometers, as the wiper arm travels over the resistance, the resistance between the wiper arm and the two ends of the potentiometer changes, although the total resistance remains fixed. Similarly, in known optical potentiometers, the photocells are varied inversely and simultaneously as the control dial is rotated in either direction. Such devices may be useful in many applications but there are many more in which it would be desirable to have only one of the photocells receive energizing light at the time.